1. Field of the Invention
The present invention relates to methods for producing a torsion spring for a micromechanical torsion spring/mass system. More particularly, the invention pertains to the production of a torsion spring from two wafers or wafer composites that possesses low torsional relative to transverse stiffness in the lateral and vertical directions.
2. Description of the Prior Art
Silicon torsion springs are known in various design variants in microstructuring. For example, C. Kaufman, J. Markert, T. Werner, T. Gessner and W. Dotzel in “Characterization of Material and Structure Defects on Micromechanical Scanners by Means of Frequency Analysis” Proceedings of Micro Materials (1995) p. 443, describe relatively long narrow strips, for example, for the articulated mounting of torsion mirrors. The spring cross section is of trapezoidal shape. Springs are formed on opposite wafer edges and produced by etching pits from the back surface during structuring of the springs from the front surface. J. Choi, K. Minami and M. Esahi in “Silicon Angular Rate Sensor by Deep Reactive Ion Etching,” Proceedings of the International Symposium on Microsystems, Intelligent Materials and Robots (Sendai, Japan, 1995), pages 29 through 32, propose the production of a rectangular torsion cross section, in particular for suspending a tuning fork resonator, with a relatively high aspect ratio (height:width ratio ≧4) by deep RIE (reactive ion etching). The two torsion spring cross sections have the disadvantage of sensitivity to transverse stresses. The spring cross-section produced by the first method is particularly sensitive to vertical bending, while the spring cross section produced by the second method is particularly sensitive to lateral bending.
German patent document DE 28 18 106 A1 discloses a torsion spring of cross-shaped cross-section that has low torsional compared to transverse rigidity in the lateral and vertical directions. A tube for a sensor that also acts as a torsion spring is disclosed by P. Enoksson et al. in “A Silicon Resonant Structure for Coriolis Mass-Flow Measurements,” Journal of Microelectromechanical Systems, Vol. 6, No. 2 (June 1997) at pages 119 through 125. The tube is produced using the Coriolis principle, by turning wafers, placing them against one another in a mirror-symmetrical manner and bonding them, in each case with a trench formed therein.
In a further application of such torsion springs, rotary mirrors and micromechanical rotation rate sensors are mentioned in International patent publication WO 96/38710. In particular, FIG. 8 of that document illustrates a double-layer vibratory structure that is held in a frame by a cross-shaped spring joint formed from the wafer layers. This cross-shaped spring joint, which is formed from a total of four individual spring elements, improves stiffness in the wafer levels, a fact referred to in the patent publication. For a vibratory structure of this type, in which the vibrators, arranged in plate form above one another, form a micromechanical rotation rate sensor based on the Coriolis principle, it is desirable to optimize the cross-shaped spring joint in such a way that transverse stiffness relative to torsional stiffness is as high as possible in the direction of the wafer planes and perpendicular thereto (i.e. in the lateral and vertical directions).